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Author Topic: AFMA Accuracy vs. Precision  (Read 4096 times)

neuroanatomist

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Re: AFMA Accuracy vs. Precision
« Reply #15 on: May 13, 2013, 04:20:27 PM »
I may be missing something, but I am perplexed by the suggestion of having your body calibrated to the lens. If you have several lenses which are off at varying +/- values, then should we have a dedicated body for each lens?

I assume you're referring to the OP's statement about 'factory calibration of body to lens.'  That's not really what Canon Service does. Rather, if you send in a body they adjust it to match a 'reference' lens (presumably one dead center in the tolerance range), and if you send in a lens they adjust it to a 'reference' body of the type you have.  If you send body and lens, they adjust both to the reference counterpart, not to each other.

That's right. If you send the pair to Canon for calibration then they calibrate for just that pair, and if you have other lenses then they will be correctly matched only if they have the same focus as the calibrated lens. But, if you calibrate the the AFMA yourself, then you can do it for all your individual lenses.

A key difference is that Canon can make more complex adjustments to a lens than AFMA.  For example, they can adjust a zoom lens at several focal lengths, not just one for the whole zoom range (or the ends with interpolation on recent bodies) as we do with AFMA.
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Re: AFMA Accuracy vs. Precision
« Reply #15 on: May 13, 2013, 04:20:27 PM »

RGF

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Re: AFMA Accuracy vs. Precision
« Reply #16 on: May 14, 2013, 03:04:41 PM »
Hello Everyone,

Just wanted to pose a question to those with sufficient experience AFMA testing and adjusting several lenses and bodies.  Specifically, I want to discuss Accuracy (how 'spot-on' focus is relative to a target) and Precision (the repeat-ability of focus).

Ref: http://www.the-digital-picture.com/Photography-Tips/AF-Microadjustment-Tips.aspx

I have found that more often than not, zero AFMA adjustment on a body for a given lens yields more precision (repeatable results) even if the depth of focus is "off" (front or back) rather than once you adjust the AFMA in either direction, even by +/- 1.  This is why I am starting to believe factory calibration of body to lens is better than AFMA tweaks.

Sometimes, but less frequently, higher precision (repeat-ability) is found on an AFMA setting other than zero, but it isn't always the "best" AFMA offset in terms of image accuracy/sharpness.

I believe this is why the FoCal AFMA testing application has a AF Consistency test, which allows you to test various AFMA values in a repeatable fashion, to seek-out the best AFMA setting with the most repeatable results.

Have any of you found the same to be true?  That sometimes you may want to trade accuracy for better precision (repeat-ability).

Thanks


Good discussion.  I have not yet tried to AFMA my cameras but I agree I want the best setting to get the maximally sharp pictures all the time.

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Re: AFMA Accuracy vs. Precision
« Reply #16 on: May 14, 2013, 03:04:41 PM »